you can use any absorbent natural fiber, either french terry or fleece or even sherpa. I don't own any flips or inserts (DD is well past the diapering stage) but those would be my best bets. I'm really leaning towards french terry because its usually cheaper than fleece but out of the 3 sherpa is the cheapest. If the inserts are super "fluffy" feeling then its probably french terry

french terry is a different weave. think of sweat pants...that's a type of french terry. and even then the way its knit together french terries look different also. but I'm almost certain that the flip inserts are from french terry. its thinner/less bulky than fleece but absorbent.